1. Field of the Invention
The present invention relates to an electric power steering apparatus for converting the rotational motion of a steering assist motor into the linear motion of a steering shaft by using the ball screw mechanism which converts rotational motion into linear motion or converts linear motion into rotational motion.
2. Description of Related Art
Ball screw mechanisms for converting the rotational motion of a rotating drive source, such as an electric motor, into linear motion or conversely converting linear motion into rotational motion are used in various industrial fields. For example, a ball screw mechanism is used in an electric power steering apparatus having a steering assist motor and driven in accordance with steering operation. This kind of electric power steering apparatus uses a ball screw mechanism wherein a female thread groove formed around the inner circumference of a cylindrical ball nut rotated around the axis thereof by a steering assist motor is meshed with a male thread groove formed around the outer circumference of the rack shaft (steering shaft) of a rack-and-pinion steering screwed via a plurality of balls, whereby the rotational motion of the steering assist motor is converted into linear motion in the axial direction of the rack shaft (for example, see Japanese Patent Application Laid-open No. 2001-122136.).
FIG. 1 is a schematic cross-sectional view, taken in the axial direction, showing the shape of the screwing portion of a conventional ball screw mechanism. Reference numeral 11 denotes a cross-section of a rack shaft, and reference numeral 13 denotes a cross-section of a ball nut. A male thread groove 11a formed around the outer circumference of the rack shaft 11 is screwed with a female thread groove 13a formed around the inner circumference of the ball nut 13 via a plurality of balls 14 (only one ball is shown in FIG. 1). Portions (contact portions) of the cross-section, where the male thread groove 11a and the female thread groove 13a contact with the ball 14, are each formed in a composite round shape (gothic arc shape) that is obtained when two discrete round shapes are combined so as to intersect at the bottom of the thread groove.
In the conventional ball screw mechanism having the thread grooves formed in such composite round shape, in the case when the clearance between the ball 14 and each of the male thread groove 11a and the female thread groove 13a is made smaller, the ball 14 makes contact with the thread grooves at four points. More specifically, the ball 14 makes contact with each of the thread grooves 11a and 13a at two points, respectively. This has a problem of causing skew. In addition, it is difficult to form the right and left two discrete round shapes, which are combined to form the composite round shape at each of the thread grooves 11a and 13a, having exactly the same dimensions and shape. This has a problem of being apt to cause dimensional errors.